This invention relates generally to navigation of vessels, and more specifically, to determination of and compensation for heave and heave rates for vessels.
Heave is the vertical distance of motion relative to sea-level. and a vertical motion of a vessel riding waves at sea, relative to sea level, is generally referred to as heave rate. For a vessel riding the waves, the vertical motion is simply an oscillation about sea-level, having a zero long-term average of earth-relative displacement and velocity. Heave and heave rate values are utilized in controlling various operations, such as, to control a tether to a submerged diver, submarine, and for other underwater operations, as well as to define a motion of a sonar transmitter, and to define deck motion at a point for landing operations of a helicopter.
Some known inertial systems are used to measure heave and heave rate. However, a vertical channel of a pure inertial system typically has divergent errors. A global positioning satellite (GPS) system can be used to compensate for such divergence, but the GPS system itself may introduce errors into the heave and heave rate determinations.
In one aspect, a method for determining heave and heave rate for a vessel is provided. In an exemplary embodiment, the vessel is equipped with an inertial navigation system (INS), and sensors for the INS are located at a selected point of the vessel, e.g. point A. The vessel further has a zero heave reference point B. The method is utilized to determine heave and heave rate at a point C of the vessel, where point C is a different point than points A and B. More particularly, the method comprises determining reference coordinates for points A, B, and C of the vessel, generating a signal representative of velocity at a point on the vessel, generating a heave rate signal based upon the velocity signal, and generating a heave signal based upon the heave rate signal.
In another aspect, an inertial navigation system (INS) for determining a heave and a heave rate for a vessel is provided. The INS system comprises, in an exemplary embodiment, a main unit, a user interface, a global positioning satellite (GPS) receiver, and a sensor unit. The sensor unit comprises an inertial sensor assembly located at a point A of the vessel. The main unit comprises interfaces for communication of inertial navigation information to other control systems on the vessel. The INS is configured to determine reference coordinates for points A, B, and C of the vessel, a heave, and a heave rate for the vessel.
In a further aspect, a filter is provided which comprises a first stage and a second stage. The first stage is configured to generate an output signal representative of a heave rate based, at least in part on a velocity signal input. The second stage is configured to generate an output signal representative of heave by filtering the heave rate signal output by the first stage.